1. Field of the Invention
The present invention relates to a camera system suitable for a TV camera, a video camera, or a 35-mm film photographic camera. More particularly, this invention is concerned with a camera system having a rear-focus zoom lens made up of a plurality of lens units capable of moving independently during, especially, zooming or focusing.
2. Related Background Art
As for zoom lenses employed in TV cameras, video cameras, or photographic cameras, a variety of rear-focus zoom lenses have been proposed in the past. Herein, a lens unit other than the first lens unit near an object is moved for focusing.
In general, the effective diameter of the first lens unit in a rear-focus zoom lens is smaller than that in a zoom lens in which the first lens unit is moved for focusing. This permits a compact lens system and simplifies proximity photography, especially, super proximity photography. Furthermore, since a relatively small and lightweight lens unit is moved to achieve focusing, only a small drive force is needed to drive the lens unit. Thus, quick focusing is realized.
FIG. 5 is a schematic diagram showing an optical system centered on a so-called rear-focus zoom lens in which a lens unit 55 or part of a relay lens 53 positioned behind a conventional zooming system or all lens units 53 are moved to perform focusing.
In FIG. 5, a lens unit 51 and a lens unit 54(R) are stationary. A lens unit 52 (V) serving as a varifocal lens changes its position in the optical-axis direction to adjust a focal length. A lens unit 55 (RR lens) or part of a relay lens 53 not only corrects variation of the image plane during zooming but also adjusts focus.
FIG. 6 is an explanatory diagram concerning the zoom lens shown in FIG. 5, wherein the x axis represents the positions of the lens unit 52 (V) or zoom positions and the Y axis represents the positions of the lens unit 55 (RR lens) during focusing.
In the lens configuration shown in FIG. 5, if a subject distance changes or the focal length (zoom position) of the zoom lens varies, the optical-axis position of the lens unit 55 (RR lens) must be changed.
A rear-focus zoom lens having the foregoing configuration has been proposed in, for example, Japanese Patent Publication No. 52-15226 (U.S. Pat. No. 4,043,642). According to this publication, a sensing means is employed to sense the optical-axis positions of a zooming lens unit and a focusing lens unit serving as a compensator. Using the positional information of both the lens units, an arithmetic logic means calculates the optical-axis position of the focusing lens unit at which focusing is attained. Based on the result of the calculation, the focusing lens unit is driven and controlled using a motor.
Japanese Patent Application Laid-Open No. 60-143309 has proposed a zoom lens in which the optical-axis positions of a varifocal lens unit and a compensator lens unit having a focusing function are stored in a storage means in association with the positions of the varifocal lens unit. According to the position of the varifocal lens unit, the position of the compensator lens unit is read from the storage means and thus the compensator lens unit is driven and controlled. The related art disclosed in U.S. Pat. No. 4161756 is well-known.
In a rear-focus zoom lens, generally, the optical-axis position of a focusing lens unit varies, as shown in FIG. 6, depending on a zoom position despite a constant object distance. Therefore, it becomes very important how quickly and precisely the focusing lens unit can be driven and controlled according to the zoom position.
A zoom lens proposed in Japanese Patent Publication No. 52-15226 performs zooming, interprets positional information of lens units, then calculates the optical-axis position of a focusing lens unit. Therefore, the processing time tends to increase.
A zoom lens proposed in Japanese Patent Application Laid-Open No. 60-143309 detects the position of a varifocal lens unit, then interprets the positional information, then reads intended information from lens position information stored in a storage means. This results in an increasing time for focusing. In the range covering a focal length of a zoom lens and an object distance, the position of the focusing lens unit is fixed. This deteriorates precision in lens positioning. To improve positioning precision, storage size of a storage means must be increased.